Adjusting the height of a router bit when the router is fixed in a table is frequently difficult. Doing so is particularly difficult when using a plunge router because a plunge router is not well designed for making small adjustments in the projection of the router bit by moving the router body relative to the base. Furthermore, plunge routers are designed to be used upright, and they typically include springs that balance the router weight so that the router body will not drop uncontrollably toward the base when the body and base are unlocked. When the router is upside down in a router table, the router body and base are urged apart from each other by the force of gravity and/or springs, introducing further grief in achieving desired adjustments in bit location.
Many plunge routers have a threaded adjusting rod attached to the router base and passing through or adjacent to a ledge or other structure that is part of, or is attached to, the router body. A nut or pair of locking nuts positioned on this adjusting rod and bearing against the ledge serves to fix the position of the router base relative to the body. After-market devices for adjusting the position of the base are available in which an internally threaded nut is attached to one end of a tube and a knob is attached to the other to tube end (where the knob is sufficiently clear of the router body to be readily accessible, at least when the router is an upright position). The nut is threaded onto the adjustment rod, and rotation of the knob rotates the nut around the rod and against the ledge. The knob on such devices can be relatively inaccessible, however, when the router is mounted under a router table, and its rotation moves the router body relative to the router base too slowly when substantial changes in router bit position are being made but too quickly for truly accurate adjustments because of the typically coarse pitch of the adjusting rod thread.
As a result, there is a substantial need for a mechanism for securing and conveniently and accurately adjusting the position of a plunge router when it is mounted upside down in a router table so that the projection of the router bit above the table can be closely controlled.